It has long been known in the art to detect the presence of human beings and animals by detecting the heat radiation that they emit. The devices that have generally been used for the purpose contain an infrared-sensitive detector of some kind, generally one of the types pyroelectric, thermopile detector, certain semiconductor detectors such as lead sulphide detectors, or similar. These detectors are either insensitive, expensive, require cooling, or draw continuous current during their use.
An alarm detector which functions in such a manner that it draws very little or no current when it does not sense any signal, which does not require cooling, has low inherent noise, and is inexpensive, would find wide application. One consequence of these properties is that it need not necessarily be connected to the mains even if it operates for years.